Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a semiconductor wafer. The radiation-patterning tool can be, for example, a photomask or a reticle, with the term “photomask” being sometimes understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” being sometimes understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting the claims that follow, the terms “photomask” and “reticle” will be given their historical distinction such that the term “photomask” will refer to a patterning tool that defines a pattern for an entirety of a wafer, and the term “reticle” will refer to a patterning tool that defines a pattern for only a portion of a wafer.
Radiation-patterning tools contain light restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer is provided with a layer of photosensitive resist material commonly referred to as photoresist. Radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers the mask pattern to the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and a decrease in the dimensions of conductor elements which connect those integrated circuit devices. The demand for ever smaller integrated circuit devices brings with it demands for ever-decreasing dimensions of structural elements on radiation-patterning tools, and ever-increasing requirements for precision and accuracy in radiation-patterning with the tools.
An exemplary prior art radiation-patterning tool 12 is shown in FIG. 1. Radiation-patterning tool 12 comprises a substrate 14 which is at least partially transparent to radiation which is to be patterned, and a structure 16 joined to substrate 14 and formed of a material which is less transparent to the radiation than is substrate 14. Substrate 14 typically comprises fused silica (for example, quartz), and structure 16 typically comprises chrome.
FIG. 1 further illustrates radiation 18 being directed toward radiation-patterning tool 12, and shows a plot 20 of radiation intensity exiting from radiation-patterning tool 12. Plot 20 illustrates that structure 16 has attenuated the radiation intensity. Specifically, plot 20 comprises a region 22 of decreased intensity where radiation 18 has been at least partially blocked by structure 16, and higher intensity regions 24 where radiation 18 has not been blocked by structure 16. In particular embodiments of the prior art, structure 16 will comprise a material substantially opaque to radiation 18 (for example, chrome can be opaque relative to ultraviolet light), and substrate 14 will be substantially transparent to the radiation (for example, quartz can be transparent to ultraviolet light).
A problem associated with the radiation-patterning described with reference to FIG. 1 can be in accurately and reproducibly forming the dip in radiation intensity shown at region 22 of plot 20. Specifically, if radiation 18 is slightly defocused from an optimal focus position, the depth of region 22 (i.e., the change in intensity between region 22 and regions 24) can be altered, which can cause variation in a critical dimension of openings ultimately patterned into photoresist. Also, the shape of the intensity profile in graph 20 can be less precise than is desired. Specifically, it would be ideal if the intensity profile of plot 20 exactly mirrored the pattern defined by structure 16 (i.e., if the intensity profile had sharp corners at transitions between regions 24 and 22, and if region 22 had a flat bottom with a width corresponding to that of structure 16).
An improved prior art radiation-patterning tool 12a is described with reference to FIG. 2. In referring to FIG. 2, similar numbering is utilized as was used in referring to FIG. 1, with the suffix “a” used to indicate structures shown in FIG. 2. Radiation-patterning tool 12a is similar to the patterning tool 12 of FIG. 1 in that it comprises a substrate 14a which is at least partially transparent to incoming radiation 18a, and a structure 16a which is less transparent to radiation 18a than the substrate. However, radiation-patterning tool 12a differs from the patterning tool 12 of FIG. 1 in that subresolution assist features 30 are provided adjacent structure 16a. Subresolution assist features 30 are formed of an identical material as structure 16a (which simplifies processing, as a single material can be formed over substrate 14a and patterned to form features 30 and structures 16a). Features 30 are referred to as subresolution assist features because intensity variations caused by features 30 are not resolved from intensity variations caused by structures 16a at the resolution provided by the particular wavelength of incoming radiation 18a. This is shown in the intensity graph 20a. Specifically, graph 20a shows a dip 22a corresponding to a region wherein an intensity variation is caused by structure 16a, and shoulders 32 corresponding to regions wherein intensity variation is caused primarily by features 30. Since the intensity variations caused by features 30 are shoulders 32 along region 22a, rather than distinctly resolved elements, such intensity variations are subresolution variations.
Subresolution assist features 30 can alleviate some of the problems described above as being associated with the radiation-patterning tool 12 of FIG. 1. Specifically, subresolution assist features 30 can stabilize an intensity difference between non-blocked regions 24a and blocked region 22a relative to subtle variations in focus of radiation 18a. Further, subresolution assist features 30 can improve the overall shape of blocked region 22a in the intensity profile 20a relative to the shape of region 22 in intensity profile 20 of FIG. 1. Specifically, subresolution assist features 30 can flatten a bottom of region 22a, and sharpen the transition at corners of region 22a, such that region 22a has a width which better approximates a width of structure 16a than the width of FIG. 1 region 22 approximates a width of structure 16.
A problem associated with the formation of subresolution assist features is that as the dimension of semiconductor devices becomes smaller the desired dimension of subresolution assist features also becomes smaller. It is therefore becoming increasingly difficult to form satisfactory subresolution assist features as integrated circuit device dimensions decrease. It would accordingly be desirable to develop alternative methods of forming subresolution assist features.